Equidae is a family of odd-toedungulatemammals of horses and horse-like animals. It is sometimes known as the horse family. All extant equids are in the genusEquus and include the three basic groups of horses, asses, and zebras. There are eight to ten extant species in Equidae, depending on the taxonomic scheme.

The extant horses (subgenus Equus) include the domestic horse (E. caballus) and Przewalski's horse. At one point, Przewalski's horse was considered extinct in the wild, until its reintroduction from zoos into the Mongolian steppe in the 1990s. It is sometimes considered a separate species, E. przewalskii, but also is listed as a subspecies of the domestic horse or as the last remaining wild horse (E. ferus), as the subspecies E. ferus przewalskii. The tarpan (Equus ferus ferus) became extinct in the early twentieth century.

The extant asses (subgenus Asinus) include the donkey or ass (E. asinus), African wild ass (E. africanus), onager (E. hemionus), and kiang (E. kiang). The domesticated donkey often is placed as a subspecies of its presumed wild ancestor, the African wild ass, which itself is variously designated as E. africanus or E. asinus.

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With a fossil history tracing back to the early Eocene, 54 million years ago, numerous fossil equids are known. The intersection of equids with humans since the Paleolithic is evident in the drawings on cave walls. The subsequent domestication of the horse and donkey were watershed moments in humanity's culture and agriculture.

Characteristics

Equidae belongs to the order Perissodactyla, the odd-toes ungulates. In addition to equids, other odd-toed ungulates include the tapirs (Tapiridae) and the rhinoceroses (Rhinocerotidae). In members of Perissodactyla, the central axis of the foot passes through the third toe. In rhinoceroses, the first and fifth toes are lost and the animal walks on the remaining three toes. Tapirs have four toes on their front feet and three on their hind feet. In extant equids, only the third toe remains and supports the whole weight of the animal.

Extant equids are medium to large mammals, with long heads, and necks with a mane. Their legs are slender and end in the single, unguligrade toe, protected by a horny hoof. They have long, slender, tails, either ending in a tuft, or entirely covered in flowing hair. They are adapted to generally open terrain, from plains and savannas, to mountains or deserts.

The pinnae ("ears") of equids are mobile, enabling them to easily localize the origin of sounds. They have two-color, or dichromatic vision. Their eyes are set back far on the head, giving them a wide angle of view, without entirely losing binocular vision. Equids also have a vomeronasal organ, that allows males to use the flehmen, or "lip-curling" response to assess the sexual state of potential mates.

Equids are herbivores, and feed predominantly on tough, fibrous food, such as grasses and sedges. When in need, they will also eat other vegetable matter, such as leaves, fruits, or bark, but are normally grazers, not browsers. Unlike ruminants, with their complex stomachs, equids break down cellulose in the "hindgut" or cecum, a part of the colon. This is known as hindgut fermentation. Their dentition is almost complete, with cutting incisors to crop food, and grinding molars set well back behind a diastema. The high-crowned molars with complex ridges is effective for grinding grasses with high content of fibers (Moehlman 2004). The dental formula for equids is:

3.1.3-4.3

3.1.3.3

Wild equids are found in sub-Saharan Africa, Arabia, Middle East, Central Asia, and Mongolia; no wild equids are found in North America, only feral (formerly domesticated) animals (Moehlman 2004).

A feral horse herd in the western United States

Equids are social animals, living in herds or bands. Horses, along with plains and mountain zebras, have permanent herds generally consisting of a single male and a band of females, with the remaining males forming small "bachelor" herds. The remaining species have temporary herds, lasting only a few months, which may be either single-sexed or mixed. In either case, there are clear hierarchies established among the individuals, usually with a dominant female controlling access to food and water resources and the lead male controlling mating opportunities.

Females, usually called mares in horses and zebras, or, in the case of asses and donkeys, jennys, usually bear a single foal, after a gestation period of approximately 11 months. Young equids are able to walk within an hour of birth, and are weaned after 4 to 13 months. (Animals living in the wild naturally wean foals at a later date than those under domestication.) Depending on species, living conditions and other factors, females in the wild may give birth every year or every other year (Macdonald 1984).

Equids who are not in foal generally have a seasonal estrous cycle, from early spring into autumn. Most females enter an anestrus period during the winter and thus do not cycle in this period. The reproductive cycle is controlled by the photoperiod (length of the day), with estrus triggered when the days begin to lengthen. Anestrus prevents the female from conceiving in the winter months, as that would result in her foaling during the harshest part of the year, a time when it would be more difficult for the foal to survive (Ensminger 1990). However, equids who live near the equator, where there is less change in length of day from season to season, have no anestrus period, at least in theory (Eilts 2007). Further, for reasons that are not clear, about 20 percent of domestic mares in the Northern Hemisphere will cycle the year round (Eilts 2007).

Evolution

Hyracotherium, also called "eohippus".

The oldest known equid fossils date from the early Eocene, 54 to 55 million years ago. This species, Hyracotherium (formerly known as Eohippus), was a fox-sized animal with three toes on its hind feet, and four on the front feet. It was herbivorous browser on relatively soft plants, and already adapted for running. The complexity of its brain suggests that it was already an alert and intelligent animal (Palmer 1999). Later species had less toes and teeth more suited for grinding up grasses and other tough plant food.

The Equidae group became relatively large during the Miocene, with many new species appearing. By this time, equids were more truly horse-like, having developed the typical body shape of the modern animals. Many of these species bore the main weight of their bodies on their central, third, toe, with the others becoming reduced, and barely touching the ground, if at all. The surviving modern genus, Equus, had arisen by the early Pleistocene, and spread rapidly though the world (Savage and Long 1986).

Classification

Przewalski's horse, the only remaining type of "wild" horse that has never been domesticated

A domesticated horse

A donkey

An onager, one type of wild ass

A pair of Kiangs

Plains zebra

Order Perissodactyla (In addition to Equidae, Perissodactyla includes four species of tapir in a single genus, as well as five living species (belonging to four genera) of rhinoceros.) † indicates extinct species.

Cross-breeds

a mule

Przewalski's horse and the domestic horse are the only equids that crossbreed and produce fertile offspring, although Przewalski's horse has 66 chromosomes while the domestic horse has 64. The offspring possess 65 chromosomes, but are not sterile and subsequent crossings can lead back to either species. Bell (2004) notes that "if the offspring is crossed back to a domestic horse, the resulting animal will have 64 chromosomes and very few Przewalski characteristics." Because these two animals can crossbreed, some consider the Przewalski's horse as a subspecies of the domestic horse (Equus caballus) as Equus caballus przewalskii. Other consider it a species, Equus przewalskii, while others consider it the last remnant of the wild horse (Equus ferus) as the subspecies Equus ferus przewalskii.

Other species of equidae can crossbreed but the ensuing offspring are usually infertile. Some hybrid equidae include:

Mule, a cross between a male donkey and a female horse. Mules are the most common type of hybrid equid and are renowned for their toughness, surefootedness, and working ability.

Hinny, a cross between a female donkey and a male horse. Considered a less desirable cross than a mule, generally smaller in size and not as hardy.

International Commission on Zoological Nomenclature (ICZN). 2003. Opinion 2027 (Case 3010). Usage of 17 specific names based on wild species which are pre-dated by or contemporary with those based on domestic animals (Lepidoptera, Osteichthyes, Mammalia): Conserved. Bull.Zool.Nomencl. 60: 81-84.

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